1. Field of the Invention
This invention relates to a car-speed control system used for controlling the travelling speed of a motor vehicle at a set point automatically.
2. Description of the Prior Art
Heretofore, there have been various types of such car-speed control systems. Among them, for example, there has been a car-speed control system having circuit formation as shown in FIG. 3.
A car-speed control system 101 shown in FIG. 3 is provided with a car-speed sensor 102 for generating a speed signal in response to an actual car-speed, a command switch 103 for outputting a cruise command signal, an actuator 104 for driving a throttle 105 by rotation of a driving motor 104b through an electromagnetic clutch 104a, a driving circuit 106 for rotating the driving motor 104b of the actuator 104 in the forward or reverse direction, a clutch-driving circuit 107 for changing the electromagnetic clutch 104a of the actuator 104 off and on electromagnetically, a detecting circuit 108 for detecting voltage level of the clutch-driving circuit 107 when the electromagnetic clutch 104a of the actuator 104 is in an unexcited state, and housed with a car-speed memory for memorizing the speed signal of the car-speed sensor 102 in response to an operation (ON-operation or OFF-operation) of the command switch 103 and a controller for controlling the driving circuit 106 in accordance with difference between actual car-speed and memorized car-speed and actuating the clutch-driving circuit 107 in a microcomputer 109.
The clutch-driving circuit 107 has circuit formation provided with a pnp-type transistor Tr101, a npn-type transistor Tr102, a diode D101 for passing a base current of the transistor Tr101 in one direction when the transistor Tr102 functions, a diode D102 for protecting the transistor Tr101 from back electromotive force, a diode D103 for protecting the transistor Tr102 from back electromotive force and a resistors R101 to R103, passes the base current of the transistor Tr101 and actuates the transistor Tr101 by actuating the transistor Tr102 according to a clutch-driving signal output from the microcomputer 109, and causes the electromagnetic clutch 104a of the actuator 104 to be in its excited state.
The detecting circuit 108 has circuit formation provided with resistors R105 to R107 for dropping voltage level at a node A, a diode D105 and a condenser C101, detects voltage level of the transistor Tr101 of the clutch-driving circuit 107 and inputs the voltage level into the microcomputer 109 by detecting voltage level at the node A when the electromagnetic clutch 104a of the actuator 104 is in an unexcited state.
In the car-speed control system 101, by operating a setting switch of the command switch 103, the car-speed at the time of the operation is memorized in the car-speed memory in the microcomputer 109. The controller actuates the driving circuit 106 and causes the electromagnetic clutch 104a of the actuator 104 to be in the excited state according to the difference between the memorized car-speed and the actual car-speed, thereby driving the throttle 105 in the opening or closing direction by the rotation of the driving motor 104b of the actuator 104 through the electromagnetic clutch 104a, and the car-speed is controlled into the memorized car-speed so as to travel at a desired constant speed. Depressing a brake pedal at the time of traveling in the constant speed, the state in which the car-speed is kept under control is cancelled by inputting a cancel signal generated according to the depressing operation of the brake pedal into the controller in the microcomputer 109, the rotation of the driving motor 104b in the actuator 104 is discontinued by the driving circuit 106 according to a cancel signal output from the controller, and the electromagnetic clutch 104a of the actuator 104 is made to change to the unexcited state, thereby cancelling the system.
However, in the aforementioned conventional car-speed control system 101, it is only possible to detect functional abnormality in the clutch-driving circuit 107 in the range up to input voltage level of the microcomputer 109 by the detecting circuit 108, because there is a limit in the input voltage level of the microcomputer 109 at the time of detecting the functional abnormality of the clutch-driving circuit 107 by the microcomputer 109 through the detecting circuit 108 after breaking the electromagnetic connection of the electromagnetic clutch 104a of the actuator 104. Such a state corresponds to the state in which the transistors Tr101 and Tr102 in the clutch-driving circuit 107 break down by open breakage, and there is problem since there is a possibility that it is impossible to detect the functional abnormality in the clutch-driving circuit 107 on the assumption that there is a very rarely case where the detecting circuit 108 is impressed with high voltage equivalent to the voltage level at the electromagnetically connected state of the electromagnetic clutch 104a according to trouble caused by short breakage of the transistors Tr101 and Tr102.